Thermal vacuum testing often requires temperature control of chamber shrouds and heat exchangers within the -160.degree. to +90.degree. C. range. There are several conventional methods which are normally employed to achieve control over this range. In a first method, using single-pass flow, temperature control is achieved by alternately pulsing hot gaseous nitrogen and cold liquid nitrogen into the feed line to the shroud to obtain set-point temperature. In a second method, using closed-loop circulation, temperature control is accomplished by either electrically heating or liquid nitrogen cooling the circulated gaseous nitrogen to the shroud to obtain set-point temperature. A third method, which uses a mass flow ratio controller and modulating control valves on gaseous nitrogen and liquid nitrogen lines, provides excellent control, however, equipment for this method is expensive and cost-prohibitive for all but long term continuous processes.
Unfortunately, single-pass methods in the past provided marginal control and often resulted in unexpected overcooling of the test article when even a short pulse of liquid nitrogen is delivered. Where precise temperature control is needed, i.e. about .+-.2.degree. C., single-pass systems typically have not provided the precision required, primarily because of overcooling temperature excursions.
The closed loop circulation method provides excellent control but, unfortunately, requires an expensive blower capable of operating at elevated pressures and cryogenic temperatures. Where several individual; circuits are to be controlled at different temperatures, the use of expensive cryogenic blowers for each circuit is also cost-prohibitive, especially for short duration or one-of-a-kind tests.
Accordingly, there is a need for an inexpensive single-pass system having high precision temperature control.